1. Field of the Invention
The present invention relates to a process for producing a thin film semiconductor device and a laser irradiation apparatus. The laser irradiation apparatus is used for crystallizing a semiconductor thin film by using excimer laser light in the process for producing a thin film semiconductor device.
2. Description of the Related Art
A thin film transistor used for switching pixels of an active matrix type liquid crystal display device, a thin film transistor formed in a peripheral circuit driving a switching transistor, and a thin film transistor used in a load device type static RAM employ an active layer comprising amorphous silicon or polycrystalline silicon. Since polycrystalline silicon has higher mobility than amorphous silicon, a thin film transistor of high performance can be obtained. However, because polycrystalline silicon contains non-bonding pairs of silicon atoms in a high density in comparison to single crystal silicon, the non-bonding pairs cause a leakage electric current on channel off. As a result, it becomes a cause of lowering the operation speed on switch on. Therefore, in order to improve the characteristics of the thin film transistor, it is demanded to form a semiconductor thin film of polycrystalline silicon having less crystal defects and excellent uniformity. As a process for forming such polycrystalline silicon thin film, a chemical gas phase growing method and a solid phase growing method are proposed. As means for decreasing the non-bonding pairs, which causes the leakage electric current, a hydrogenation technique is employed, in which the non-bonding pairs are terminated by doping hydrogen in the polycrystalline silicon thin film. However, when crystals having a large particle size are grown by the chemical gas phase growing method to form a polycrystalline silicon thin film, the film thickness thereof becomes uneven. Accordingly, it is difficult to form a transistor having uniform device characteristics by using the polycrystalline silicon thin film.
As a process for forming a polycrystalline semiconductor thin film considering the problems described above, an annealing treatment using excimer laser light is proposed. In this process, a non-single crystal semiconductor thin film, such as amorphous silicon and polycrystalline silicon having a relatively small particle diameter, formed on an insulating substrate is irradiated with laser light to locally heated, and the semiconductor thin film is converted into polycrystals having a relatively large particle diameter (crystallization) during the cooling process. A thin film transistor is formed by integrating using the semiconductor thin film thus crystallized as an active layer (channel region). By using the laser annealing, a thin film semiconductor device can be produced by a low temperature process, and an inexpensive glass substrate can be used instead of an expensive quartz substrate excellent in heat resistance. Furthermore, because the excimer laser light is in the ultraviolet region, which results in large absorption coefficient for silicon, it has an advantage in that only the surface of the silicon can be locally heated but the insulating substrate is not thermally damaged. A method of the laser annealing includes a first method, in which an amorphous silicon thin film is directly irradiated with excimer laser light to convert into polycrystals, and a second method, in which a polycrystalline silicon thin film formed by solid phase growth is irradiated with excimer laser light at such an energy level that the whole film is not melted to conduct annealing.
The first method, the direct annealing of the amorphous silicon thin film, is advantageous for mass production of an LSI in future since it is simple in process in comparison to the second method. Furthermore, when a large area can be subjected to the bulk annealing treatment by irradiation of excimer laser light at a time, it is further advantageous for mass production. However, in the case where a conventional laser irradiation apparatus is used for the direct annealing of an amorphous silicon thin film, it has been difficult to obtain excimer laser light that has a large area with uniform cross sectional distribution of energy at a single shot sufficient to obtain a polycrystalline silicon thin film excellent in crystallinity and having small density of grain boundary trap. In order to solve the problem, an excimer laser irradiation apparatus is being developed that has such high output energy that a large area can be subjected to the bulk annealing treatment with a single shot at a time. Furthermore, in order to improve the effect of annealing using excimer laser light, a method is proposed, in which a substrate is previously heated to several hundreds degrees centigrade, and then the direct annealing of the amorphous silicon is conducted. However, even by using the excimer laser irradiation apparatus of high output energy, process conditions for obtaining a polycrystalline silicon thin film excellent in crystallinity and having small density of grain boundary trap have not yet been established. Furthermore, in the conventional direct annealing method of amorphous silicon, the crystalline particle diameter of polycrystalline silicon obtained is 50 nm or less in average, and thus further increase in crystalline particles size is demanded. In the laser annealing process using such a laser irradiation apparatus of high output of an emission duration of 50 ns or more, the crystallization process is conventionally conducted in the air. In this case, however, crystal defects are formed by combining oxygen in the air and silicon, and therefore there is a problem in that the mobility of the thin film transistor is not so improved that expected from the particle diameter (grain size) of the polycrystalline silicon.
As for the laser irradiation apparatus emitting excimer laser light, because output energy of laser light of the conventional apparatus is small (about 0.5 J), a method has been generally employed in that a linear beam having an irradiation area of 200 mmxc3x970.6 to 0.7 mm is irradiated with an overlap of about from 90 to 95%. In this method, however, the output stability of laser is poor (about xc2x110% in the current situation), and non-uniformity of crystals is caused at a part where the output energy becomes unexpectedly large or small. When a circuit is integrated and formed on such a part, it becomes a cause of operation failure. It is also considered to employ an overlap of the laser beam of about 99% to disperse the dispersion of output as possible. However, such a method involves a problem in that the throughput becomes extremely poor to bring about increase in production cost. When the crystallization is conducted by using the conventional linear laser beam with an overlap, for example, of 95%, it requires about 6 minutes for treating a substrate of 400 mmxc3x97500 mm. When the same substrate is treated with an overlap of the linear laser beam of 99%, it requires 30 minutes. Furthermore, since the laser annealing is generally conducted in vacuum, it requires about 5 minutes for loading and unloading of the substrate.
In recent years, an excimer laser irradiation apparatus having high output energy that can conduct the annealing treatment of a large area with a single shot at a time has been developed as described in the foregoing. For example, an area of about 27 mmxc3x9767 mm can be irradiated at a time by using, for example, an excimer laser light source having output of 10 J. However, in order to produce a large area LCD panel having a dimension across corners of about 20 inches (about 120 mmxc3x97160 mm) required for a large display, a xe2x80x9cboundary partxe2x80x9d of laser irradiation is necessarily formed in either method. There is a problem in that when the other part than the xe2x80x9cboundary partxe2x80x9d is irradiated at optimum energy, the xe2x80x9cboundary partxe2x80x9d is over-irradiated, in which the semiconductor thin film is microcrystallized to deteriorate the performance of the thin film transistor.
FIGS. 1A to 1C are diagrams schematically showing the conventional laser irradiation treatment for converting amorphous silicon formed on a glass substrate to polycrystalline silicon. In the case where a semiconductor thin film formed on an insulating substrate 0 having a larger area than the cross sectional area of the laser light is irradiated, scanning of the laser light must be conducted relatively with respect to the insulating substrate 0. In this case, the once irradiated region (FIG. 1A), the twice irradiated region (FIG. 1B), and the four times irradiated region (FIG. 1C) are formed in the surface of the glass substrate, to cause unevenness in particle diameter of polycrystalline silicon. As shown in FIG. 1C, the twice irradiation region and the four times irradiated region are the so-called xe2x80x9cboundary partxe2x80x9d, in which the diameter of the individual crystal particles is different from the crystal particle diameter in the once irradiated region.
The method where a relatively large area (for example, about 2.7 cmxc3x976.7 cm) is crystallized at a time by using a laser irradiation apparatus of high output as described in the foregoing is described, for example, in JP-A-7-235490. When a laser irradiation apparatus having extremely high output can be used, the semiconductor thin film formed on the whole surface of the substrate can be crystallized at a time. By using such a method, the throughput of the laser annealing becomes a little more than 1 minute, and thus the productivity is improved by about 5 times in comparison to the method where scanning with a linear laser beam is conducted. Furthermore, it is known that because a relatively large area is crystallized at a time, good uniformity can be obtained, and the surface homology after crystallization is improved. In a practical process, however, because the substrate is exposed to the air after forming the semiconductor thin film until the laser annealing, contamination substances and dusts from the air are attached to the surface thereof, and a removal step of them is necessary. Thus, a problem arises in that the advantage of increasing the throughput by conducting the bulk annealing is spoiled. In addition, there is a problem in that there is a limit to improving the quality of the semiconductor thin film by employing the conventional simple method, in which the semiconductor thin film is formed, and then crystallization is conducted by irradiating with laser light. Furthermore, it is practically difficult to develop a laser irradiation apparatus having total output energy exceeding 10 J. Therefore, it is impossible to crystallize a semiconductor thin film formed on a large scale substrate (for example, 30 cmxc3x9730 cm or larger) at a time. For this reason, the crystallization is conducted by separating certain regions (about 2.7 cmxc3x976.7 cm) as shown in FIGS. 1A to 1C, and therefore there is a problem in that non-uniformity tends to occur at the boundary part between the laser irradiated parts. Furthermore, non-uniformity in transistor characteristics occurs due to the dispersion in energy among the shots of laser irradiation. As a result, in the case where an active matrix display device is formed by integrating and forming thin film transistors, non-uniform display occurs in the pixel array part, and decrease in operation margin occurs in the peripheral driving circuit part. Because oscillation frequency of the laser irradiation apparatus of high output used for crystallization by bulk irradiation is 1 Hz or lower, there is a tendency that the productivity thereof is not good in comparison to a laser annealing irradiation apparatus of low output having an oscillation frequency reaching several hundreds Hz. Furthermore, while a semiconductor thin film is converted from amorphous to polycrystalline by irradiation with laser light to obtain a polycrystalline semiconductor thin film, such as polycrystalline silicon as described in the foregoing, the substrate carrying the semiconductor thin film is maintained at room temperature or is heated on conducting irradiation with laser light. However, the size of crystalline particles is increased by this method, but there is a problem in that characteristics, such as a threshold voltage and an on electric current, of a TFT using a polycrystalline semiconductor thin film as an active layer tend to exhibit large dispersion.
An object of the invention to solve the problems associated with the conventional techniques described above to form a semiconductor thin film comprising polycrystalline silicon excellent in crystallinity on an insulating substrate, and additionally to provide a laser irradiation apparatus realizing the same. The invention comprising the following means to achieve the objects includes a first aspect and a second aspect. The first aspect relates to a process for producing a thin film semiconductor device comprising a film forming step of forming a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter on a substrate; an irradiation step of irradiating the semiconductor thin film with an energy beam to convert the amorphous material or the polycrystalline material having a relatively small particle diameter to a polycrystalline material having a relatively large particle diameter; and a forming step of integrating and forming a thin film transistor in a prescribed region by using the semiconductor thin film thus converted to the polycrystalline material as an active layer, wherein the irradiation step is bulk irradiation conducted in such a manner that a cross sectional shape of the energy beam is adjusted with respect to the region to crystallize the region at a time by a single shot irradiation, so that characteristics of the thin film transistor are made uniform. For example, the forming step may comprise integrating the thin film transistors to form a thin film semiconductor device for a display panel equipped with a pixel array and a scanner circuit, and the irradiation step may comprise irradiating a region, on which the scanner circuit is integrated and formed, at a time. The irradiation step may comprise making uniform threshold value characteristics of the thin film transistor contained in the region by the bulk irradiation. In this case, the forming step may comprise forming, in the region, at least one circuit selected from an operational amplifier circuit, an analog-digital conversion circuit, a digital-analog conversion circuit, a level shifter circuit, a memory circuit, and a microprocessor circuit. The first aspect of the invention further includes a laser irradiation apparatus, by which a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter formed on a substrate is irradiated with laser light to convert into a polycrystalline material having a relatively large particle diameter. The laser irradiation apparatus comprises a laser light source emitting laser light having a prescribed cross sectional shape; shaping means of shaping the cross sectional shape of the laser light to adjust to a prescribed region; and irradiating means of irradiating a semiconductor thin film with the shaped laser light to uniformly crystallize in the region. A laser irradiation apparatus for irradiating a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter formed on a substrate capable of carrying information for processing with laser light to convert into a polycrystalline material having a relatively large particle diameter is also included, wherein at least one condition selected from a cross sectional shape, an irradiating position, an energy amount, an energy distribution and a moving direction of the laser light is capable of being adjusted by reading the information. For example, the information is read by recognizing a pattern formed on a surface of the substrate. Alternatively, the information is read by detecting a code written in the substrate.
The second aspect of the invention relates to a process for producing a thin film semiconductor device comprising a film forming step of forming a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter on a substrate, on which plural units are formed; an irradiation step of intermittently irradiating the semiconductor thin film with an energy beam moving with respect to the substrate, to convert the amorphous material or the polycrystalline material having a relatively small particle diameter to a polycrystalline material having a relatively large particle diameter; and a forming step of integrating and forming a thin film transistor by using the semiconductor thin film thus converted to the polycrystalline material as an active layer, to form thin film semiconductor devices in the respective units, wherein the irradiation step is bulk irradiation conducted in such a manner that a cross sectional shape of the energy beam is adjusted with respect to the unit to irradiate one or two or more units at a time by a single shot irradiation. It also relates to a laser irradiation apparatus, by which a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter formed on a substrate, on which prescribed units are formed, is intermittently irradiated with laser light moving with respect the semiconductor thin film to convert into a polycrystalline material having a relatively large particle diameter, comprising a laser light source intermittently emitting laser light; an optical system for enlarging or reducing a cross sectional shape of the laser light to adjust to a unit; and shielding means for shielding a part other than the units from the laser light, wherein irradiation is conducted by bulk irradiation of one or two or more units at a time by a single shot irradiation. Preferably, it comprises moving means for moving the substrate with respect to the laser light to make possible to irradiate all of the units with the laser light. Furthermore, it may comprise detecting means for optically reading a positioning mark provided on the substrate, and controlling means for controlling the moving means corresponding the mark thus read.
According to the first aspect of the invention, by using a laser irradiation apparatus having output that is capable of crystallizing a region having a prescribed area or more at a time, the cross sectional shape of the laser light can be changed and adjusted to the region, in which uniformity in device characteristics is required (such as a circuit region). The cross sectional shape of the laser light can be changed to set at the optimum shape for the respective products of various thin film semiconductor devices. By conducting the crystallization of a semiconductor thin film using such a laser irradiation apparatus, uniform polycrystalline can be formed on a prescribed region. By forming thin film transistors are integrated and formed thereon, uniform device characteristics can be realized, and a high performance circuit can be stably produced in the prescribed region. According to the second aspect of the invention, because the crystallization is conducted by irradiating each the respective units, on which the thin film semiconductor devices are formed, with laser light at a time by a single shot irradiation, the xe2x80x9cboundary partxe2x80x9d of laser light is substantially not present in the units, and uniformity can be realized. According to the manners, a uniform polycrystalline semiconductor thin film having a crystalline particle diameter reaching 1,500 nm (with a dispersion of about xc2x1100 nm) having a small electron trap density in the particle boundary and inside the particles can be formed even in an active matrix type display device of a 20-inch class.
Another object of the invention is to increase productivity of laser annealing and to improve quality of a crystallized semiconductor thin film. In order to attain the objects, the following means as a third aspect have been conducted. That is, the invention relates to a process for producing a semiconductor thin film comprising a film forming step of forming a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter on a substrate; and a laser annealing step of irradiating a prescribed region of the semiconductor thin film at a time with laser light having a prescribed cross sectional area to convert the amorphous material or the polycrystalline material having a relatively small particle diameter to a polycrystalline material having a relatively large particle diameter, wherein the film forming step and the laser annealing step are alternately repeated without exposing the substrate to the air, so as to accumulate the semiconductor thin films. It is preferred that the laser annealing step comprises irradiating with laser light at a condition in that TE/(dxc2x7S) is from 0.01 to 1, wherein d (nm) represents a thickness of the semiconductor thin film having been formed, TE (J) represents total energy of the laser light, and S (cm2) represents an area of a region irradiated with the laser light at a time. It is also preferred that the laser annealing step is repeated with the laser light having such energy that is being increased along with the lapse of the steps. Alternatively, the film forming step is repeated to form a semiconductor thin film having such a thickness that is being decreased along with the lapse of the steps. The invention also relates to an apparatus for producing a semiconductor thin film comprising a film formation chamber where a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter is formed on a substrate; and a laser annealing chamber where a prescribed region of the semiconductor thin film is irradiated at a time with laser light having a prescribed cross sectional area to convert the amorphous material or the polycrystalline material having a relatively small particle diameter to a polycrystalline material having a relatively large particle diameter, wherein the apparatus further comprises means for transporting the substrate back and forth between the film forming chamber and the laser annealing chamber without exposing the substrate to the air, so as to accumulate the semiconductor thin films by alternately repeating the film forming step and the laser annealing step. It is preferred that irradiation of laser light is conducted, in the laser annealing chamber, at a condition in that TE/(dxc2x7S) is from 0.01 to 1, wherein d (nm) represents a thickness of the semiconductor thin film having been formed, TE (J) represents total energy of the laser light, and S (cm2) represents an area of a region irradiated with the laser light at a time. It is also preferred that irradiation of laser light is repeated, in the laser annealing chamber, with the laser light having such energy that is being increased along with the lapse of the steps. Alternatively, film formation is repeated, in the film formation chamber, to form a semiconductor thin film having such a thickness that is being decreased along with the lapse of the steps.
According to the third aspect of the invention, when the prescribed area of the semiconductor thin film is irradiated with laser light having the prescribed cross sectional area to convert the amorphous material or the polycrystalline material having a relatively small particle diameter to the polycrystalline material having a relatively large particle diameter, the film forming step and the laser annealing step are alternately repeated without exposing the substrate to the air, so as to accumulate the semiconductor thin films. Since the semiconductor thin film formed on the substrate is immediately subjected to laser annealing without exposing to the air, the productivity is improved, and contamination of the surface of the semiconductor thin film due to the air is avoided. Furthermore, by alternately repeating the film forming step and the laser annealing step to accumulate the semiconductor thin films, a semiconductor thin film having an extremely good crystallinity can be finally obtained.
In order to attain the objects of the invention, the following means has been conducted as a fourth aspect. That is, the fourth aspect of the invention relates to a process for producing a semiconductor thin film comprising a film forming step of forming a non-single crystal semiconductor thin film on a surface of a substrate, and an annealing step of irradiating the non-single crystal semiconductor thin film with laser light to convert to a polycrystalline material, wherein the annealing step is conducted in such a manner that the semiconductor thin film is irradiated once or more with a pulse of laser light having a constant cross sectional area and an emission time width from upstand to downfall of 50 ns or more, so as to convert the semiconductor thin film contained in an irradiated area corresponding to the cross sectional area to a polycrystalline material at a time, and an energy intensity of the laser light from upstand to downfall is controlled to apply a desired change. It is preferred that the annealing step has an inclined change in that an energy intensity at downfall is smaller than an energy intensity at upstand. Alternatively, the annealing step may have an inclined change in that an energy intensity at downfall is larger than an energy intensity at upstand. It is also preferred that when an energy density of the laser light is controlled to apply a desired change, a changing width thereof is 300 mJ/cm2 or less. It is also preferred that the annealing step comprises shaping a rectangular shape of laser light having a cross sectional area of 100 cm2 or more. The fourth aspect of the invention also relates to a laser irradiation apparatus for irradiating a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small particle diameter formed on a substrate with laser light to convert into a polycrystalline material having a relatively large particle diameter, which comprises a laser light source emitting a pulse of laser light having an emission time width from upstand to downfall of 50 ns or more; shaping means for shaping a cross sectional area of the laser light to a prescribed shape; irradiating means for irradiating the semiconductor thin film at least once with a pulse of the laser light thus shaped, so as to convert the semiconductor thin film contained in an irradiated area corresponding to the cross sectional area to a polycrystalline material at a time; and controlling means for controlling an energy intensity of the laser light from upstand to downfall to apply a desired change. It is preferred that the controlling means applies an inclined change in that an energy intensity at downfall is smaller than an energy intensity at upstand. Alternatively, the controlling means may apply an inclined change in that an energy intensity at downfall is larger than an energy intensity at upstand. It is also preferred that when an energy density of the laser light is controlled to apply a desired change, a changing width thereof is 300 mJ/cm2 or less. It is also preferred that the shaping means comprises shaping the laser light having a cross sectional area of 100 cm2 or more to a rectangular shape.
The fifth aspect of the invention relates to a process for producing a semiconductor thin film comprising a film forming step of forming a non-single crystal semiconductor thin film on a surface of a substrate; and an annealing step of irradiating the non-single crystal semiconductor thin film with laser light to convert the non-single crystal semiconductor thin film to a polycrystalline material, wherein the annealing step is conducted in such a manner that the substrate is irradiated once or more with a pulse of laser light having an emission time width of 50 ns or more and a constant cross sectional area with maintaining the substrate in a non-oxidative atmosphere, so as to convert the semiconductor thin film contained in an irradiated area corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that the annealing step is conducted in such a manner that the substrate is irradiated with the laser light with maintaining the substrate in the non-oxidative atmosphere comprising vacuum. Alternatively, the annealing step is conducted in such a manner that the substrate is irradiated with the laser light with maintaining the substrate in the non-oxidative atmosphere filled with an inert gas. In this case, the substrate is irradiated with the laser light with maintaining the substrate in the non-oxidative atmosphere filled with an inert gas at an atmospheric pressure or filled with a pressurized inert gas. It is preferred that the annealing step comprises irradiating the substrate with a pulse of laser light having a cross sectional area of 5 cm2 or more. It is also preferred that the annealing step comprises irradiating the substrate with the laser light having an energy intensity controlled to a range of from 400 to 600 mJ/cm2. The fifth aspect of the invention also relates to a laser irradiation apparatus for irradiating a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small diameter formed on a substrate with laser light to convert to a polycrystalline material having a relatively large particle diameter, which comprises a laser light source emitting a pulse of laser light having an emission time width of 50 ns or more; shaping means for shaping a cross sectional area of the laser light to a prescribed shape; maintaining means for maintaining the substrate previously having a semiconductor thin film in a non-oxidative atmosphere; and an irradiating means for irradiating the substrate maintaining in the non-oxidative atmosphere once or more with the pulse of laser light thus shaped, so as to convert the semiconductor thin film contained in an irradiated region corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that the maintaining means maintains the substrate in a non-oxidative atmosphere filled with an inert gas. It is also preferred that the maintaining means maintains the substrate in a non-oxidative atmosphere filled with an inert gas at an atmospheric pressure or filled with a pressurized inert gas. It is preferred that the shaping means shapes the pulse of laser light to a rectangular shape having a cross sectional area of 5 cm2 or more. It is also preferred that the irradiation step comprises irradiating the substrate with the laser light having an energy intensity controlled to a range of from 400 to 600 mJ/cm2.
The sixth aspect of the invention relates to a process for producing a semiconductor thin film comprising a film forming step of forming a non-single crystal semiconductor thin film on a surface of a substrate; and an annealing step of irradiating the non-single crystal semiconductor thin film with laser light to convert the non-single crystal semiconductor thin film to a polycrystalline material, wherein the annealing step is conducted in such a manner that the substrate is irradiated once or more with a pulse of laser light having an emission time width of 50 ns or more and a constant cross sectional area under conditions in that the substrate is uniformly heated, so as to convert the semiconductor thin film contained in an irradiated area corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that the annealing step comprises irradiating the substrate with laser light with maintaining the substrate in a vacuum atmosphere under conditions in that the substrate is uniformly heated. Alternatively, the annealing step may comprise irradiating the substrate with laser light with maintaining the substrate in an inert gas atmosphere under conditions in that the substrate is uniformly heated. The sixth aspect of the invention also relates to a laser irradiation apparatus for irradiating a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small diameter formed on a substrate with laser light to convert to a polycrystalline material having a relatively large particle diameter, which comprises a laser light source emitting a pulse of laser light having an emission time width of 50 ns or more; shaping means for shaping a cross sectional area of the laser light to a constant shape; heating means for uniformly heating the substrate previously having a semiconductor thin film; and irradiating means for irradiating the heated substrate once or more with the pulse of laser light thus shaped, so as to convert the semiconductor thin film contained in an irradiated region corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that the heating means comprises uniformly heating the substrate comprising glass to a range of from 300 to 450xc2x0 C. It is also preferred that the heating means comprises a heat source built in a stage, on which the substrate is placed. It is also preferred that the heating means heats the substrate with maintaining the substrate in a vacuum atmosphere. Alternatively, the heating means may heat the substrate with maintaining the substrate in an inert gas atmosphere.
The seventh aspect of the invention relates to a process for producing a semiconductor thin film comprising a film forming step of forming a non-single crystal semiconductor thin film on a surface of a substrate; and an annealing step of irradiating the non-single crystal semiconductor thin film with laser light to convert the non-single crystal semiconductor thin film to a polycrystalline material, wherein the annealing step is conducted in such a manner that the substrate is irradiated once or more with a pulse of laser light having an emission time width of 50 ns or more and a constant cross sectional area under conditions in that the substrate is cooled to a temperature lower than room temperature, so as to convert the semiconductor thin film contained in an irradiated area corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that in the annealing step, cooling is conducted at a substrate temperature lower by 50xc2x0 C. or more than the substrate temperature increased by irradiation with laser light. It is more preferred that in the annealing step, cooling is conducted at a substrate temperature lower by 100xc2x0 C. or more than the substrate temperature increased by irradiation with laser light. It is also preferred that the annealing step comprises irradiating the semiconductor thin film with a pulse of laser light having a cross sectional area of from 10 to 100 cm2. The seventh aspect of the invention also relates to a laser irradiation apparatus for irradiating a semiconductor thin film comprising an amorphous material or a polycrystalline material having a relatively small diameter formed on a substrate with laser light to convert to a polycrystalline material having a relatively large particle diameter, which comprises a laser light source emitting a pulse of laser light having an emission time width of 50 ns or more; shaping means for shaping a cross sectional area of the laser light to a constant cross sectional area; cooling means for cooling the substrate previously having a semiconductor thin film to a temperature lower than room temperature; and an irradiating means for irradiating the cooled substrate once or more with the pulse of laser light thus shaped, so as to convert the semiconductor thin film contained in an irradiated region corresponding to the cross sectional area to a polycrystalline material at a time. It is preferred that the cooling means cools the substrate to a temperature lower by 50xc2x0 C. or more than the substrate temperature increased by irradiation with laser light. It is more preferred that the cooling means cools the substrate to a temperature lower by 100xc2x0 C. or more than the substrate temperature increased by irradiation with laser light.
According to the fourth aspect of the invention, in the laser irradiating apparatus for producing a polycrystalline semiconductor thin film comprising, for example, polycrystalline silicon on an insulating substrate comprising, for example, transparent glass, reformation of the polycrystalline semiconductor thin film is conducted by changing the energy intensity of the laser light during the time from the start of irradiation to the end of the irradiation. According to the fifth aspect of the invention, in the crystallization step of the semiconductor thin film comprising, for example, polycrystalline silicon to be an active layer of a thin film transistor, irradiation of laser light is conducted in a vacuum atmosphere or in an inert gas atmosphere. At this time, irradiation is conducted once or more on the same part by using excimer laser light of high output having an emission time of 50 ns or more and an irradiation area of 5 cm2 or more, so as to conduct crystal growth of the polycrystalline silicon thin film. The bulk irradiation with laser light of high output is conducted in a non-oxidative atmosphere shielded from oxygen to previously prevent the formation of crystal defects. According to the sixth aspect of the invention, in the laser irradiation apparatus capable of crystallizing an irradiation region more than the certain area at a time, a substrate heating mechanism is attached to improve the uniformity of the characteristics and to improve the productivity. Furthermore, the substrate heating mechanism is maintained in vacuum or in an inert gas. When the substrate is heated on irradiating with laser light of high output, the uniformity of the crystals is improved, and the productivity is also improved. According to the seventh aspect of the invention, when a polycrystalline semiconductor thin film is produced, the temperature of the substrate is cooled to 10xc2x0 C. or less on crystallizing at a time by irradiating laser light of high output with an irradiation area, for example, of from 10 to 100 cm2. By controlling the temperature of the substrate in such manners, a uniform polycrystalline semiconductor thin film with suppressed dispersion can be obtained although the crystalline particle diameter is not increased.
The invention is not limited to each of the effects of the individual aspects, but is effective when the beam profile of the laser light is changed and adjusted with respect to space and time. At this time, the laser light irradiation may be conducted without exposing to the air.